With regard to vehicles in general, many different power train configurations are available. For example, the gearbox may take the form of a manually operated gearbox or an automatic gearbox. With regard to heavy vehicles, it is often desirable that they should provide the driver with as comfortable a driving experience as possible. This means, for example, that the gear changes in the gearbox should be executed automatically by a control system usually incorporated in the vehicle. Automatically operated gearboxes have therefore become increasingly common in heavy vehicles.
However, this automatic gear changing is usually not executed by an automatic gearbox in the traditional sense, but by control system operation of a “manual” gearbox, partly because manual gearboxes are substantially less expensive to manufacture, but also because they are usually more efficient. With regard to automatic gearboxes of the type commonly used in passenger cars, the level of efficiency is often too low compared with a manually operated gearbox to justify their use other than in, for example, local buses and delivery vehicles in urban areas where frequent starting and stopping is usually required.
With regard to heavy vehicles largely used on major roads/motorways, automatically operated “manual” gearboxes are therefore commonly used.
This gear changing may be effected in several different ways, in one of which a clutch pedal is used to set the vehicle in motion from stationary but all other gear changing can be effected by the vehicle's control system without involving the clutch at all. Instead, the gear changes are carried out “torque-free”, i.e. the torque delivered from the engine is adjusted to a suitable level to reduce the torque transmitted at the engagement points of the relevant gears.
Another method is to use instead an automatically controlled clutch with automatic upshifts and downshifts, in which case the driver has access to only an accelerator pedal and a brake pedal.
On this type of vehicle with an automatic clutch, it is however important to feel the clutch's contact point, i.e. the position at which the clutch begins to transmit torque. The contact point is not a fixed point, but varies, mainly depending on the clutch's temperature, but also on other factors. The contact point may therefore change when the vehicle is in motion, resulting in a need to be able to estimate the contact point's position at regular intervals when the vehicle is in motion.
By feeling the contact point's position, e.g. when setting in motion or halting the vehicle or changing gear, the vehicle's control system therefore recognises the clutch position at which torque transmission from the engine to the rest of the power train begins or ceases, so that setting the vehicle in motion and gear changing can also be carried out in such a way as not to cause unwanted jerking or unwanted wear in the power train.
Estimation of the contact point's position should take place when the gearbox is in neutral position if the vehicle is not to be affected by the estimation. An example of how this can be carried out is by maneuvering the clutch from open position to closed position while the gearbox input shaft is stationary. The contact point is then determined as the first position at which a rotation speed sensor on the gearbox input shaft registers a speed.
A problem with this type of contact point estimation is that the time it takes may be too long for it to be unnoticed when the vehicle is in motion. Also, the rotation speed sensor on the gearbox input shaft is often of a type which cannot detect speeds below a certain speed, which means that it is not the actual contact point which is detected, but rather the point at which the clutch is at a position where the gearbox input axle has reached a high enough rotation speed to be detectable by the speed sensor.